I was asked recently why I called a particular character "Rowan" - was it for the tree? The woman who asked the question is into what you might call spiritual botany - which I am not - and somehow, this meant I had to give her a practical answer, because although creativity can be a mysterious force, it is also an ordinary business of putting things together and hoping they will stick.

The Rowan in question is a small boy who appears at the end of my last novel, The Gathering. He has, or I hope he has, a kind of iconic or uncanny quality. So I said that, yes, I took the name from the tree and yes, I am aware it was considered a magical tree, and a protective one in the Celtic tradition, and that this mattered a little, though not as much as the simple fact that the tree is native to Ireland. As a name, Rowan has modern, slightly hippyish overtones; it is sometimes chosen over the Gaelic "Rohan", and both these facts suited my purposes very well.

Checking my notes, I found that the little trees in the asylum graveyard that features in the book were rowans, and this synchronicity was very satisfying to me. Finally, I have met a couple of Rowans, both children, and they are both pretty fab. So yes, he was named for the tree. Sort of. But there was no magic to the naming - or not the kind of magic that my questioner was into. I chose this name for reasons poetical, tribal, sociological, personal and arbitrary. I chose this name because he is a beautiful little boy whose extended family look at him as though they had never seen a child before.

"Rowan," they say. "Rowan?"

Naming is nice. It took me days before I was able to speak a name for my first child (what if people did not like it?), and I suspect we gave her a secret, second name as well, to keep her safe. Of course, we did no such thing. We just put a word on this new human creature and used it freely and easily, the way you do. We told it to the government. We bandied it about. By the time the second child was named, we had steeled ourselves into a state of insouciance.

Naming characters is much less fraught - you know a lot more about your characters than you do about your babies, for a start. I use a dictionary of names that I bought many years ago, trying not to look pregnant at the till. I also have a dictionary of Irish surnames, to check the provenance and the meaning of a particular family name. I use the phone book, and I ask people, all the time, where their name comes from, which in some societies, particularly multicultural ones, is considered rude. I hope I am not rude, but I do want to know their story - surnames can tell you about wealth or class, but more usually they speak of movements and migrations, both ancient and modern.

In more static societies, like Ireland, you can tell where a person is from by their surname, or where their grandparents are from. So provincial, I know; so ugly, when you use it to guess their religion (but I like doing that, too). The information a name contains is both private and general. It intimates much, but it doesn't mean a goddamn thing - as any Catholic called Billy will be quite happy to tell you.

If surnames yield history, first names give us love stories: they speak of the family mythology, the kind of person your parents hoped you might become. It is important to keep this element of hope when you name a character. It is important to keep your creation open, though it is a brave writer who leaves them entirely blank (by calling his protagonist John, for example, or even "K"). A funky name can add various things to a character. I might ramp one up for poetic resonance, another writer will do it for satirical effect. All of which has to be done with care.

Though John means nothing and Paul means very little, John Paul will be a Catholic (religion again) born in the early 1980s; he will also possibly be working class. The more socially legible the name, the more satirical the book (or proud, sometimes), and the less the name is of use to me. Contempt or pride are not emotions I have about the people I create. That is not what I create them for. I create them to see what they will do.

So.

There he is. I can see him - if I stay still long enough. What I have first is a glimpse; a flicker of emotion. I know what this child does to the people around him. I spend some time trying to decide on a name, and meanwhile, by sleight, I steal some details; the colour of his hair and - goodness yes - his blue eyes. There he is. The child has turned around. What he wears is up to me, I dress him myself. No, not lace-up shoes - cute but too impractical - he wears sneakers with lycra fastenings. He keeps changing size, though - like ET in the movie - the proportions shift, depending on where, and how important, he is in the scene.

"Where do you get your ideas from?" - that is one of the great questions. To which the answer may be that there is a problem in the book, and this problem becomes a door. Through this door walks the necessary thing - in this case a child, or the sense of a child. If I stay very still, I will realise how much I already know about this human being, the rest I just make up. In between that first gift and the cutting and pasting that follows, comes the name.

It is one of the reasons I like signing books. "What's your name?" I say. I am always happy to sign for a Bernadette - named for the film The Song of Bernadette, and born in 1943. Your average Martina is born in 1961, Dolores may indeed have a sad tale to tell, Saffria will confirm that her parents were a bit nuts, but John Paul - well he will seldom declare himself. He is 28, he is buying the book for his girlfriend, or worse, his mother, and "What's her name?" I say.